Deep trench capacitors are used as components in dynamic random access memory (DRAM) devices and other integrated circuit devices. In the formation of integrated circuit devices on semiconductor wafers (chips), there is a continual desire to increase the number of devices per unit area of the chip surface. To meet this demand for increased device density, continual efforts are made to reduce the surface area taken up by device components such as capacitors. Trench capacitors having geometries involving high aspect ratios (depth normal to the principal wafer surface width parallel to the principal wafer surface) allow more capacitors can be placed on the chip.
The general fabrication of trench capacitors and other trench-based component is well known. Typical processes involve etching of an initial trench into the substrate (or wafer, usually a silicon wafer). Prior to trench etching, one or more conformal dielectric material layers may be formed on the wafer surface. Typically, a conformal oxide dielectric layer will be closest to the semiconductor surface with a conformal nitride dielectric layer (so-called "pad nitride") overlying the oxide layer. The trench is then formed by etching through the pad dielectric layers and into the semiconductor substrate material. A region in the substrate at the bottom of the trench may be doped to provide a region of increased charge storage capacity which will become one plate of the capacitor. A thin node dielectric layer (high dielectric constant material) is formed as a conformal layer covering the trench surface. The trench is then filled with a doped polycrystalline silicon (polysilicon) or other charge storage material to form the other plate of the capacitor.
In order to enhance the reliability of the trench capacitor design by minimizing parasitic leakage at the trench sidewalls, a "collar" oxide may be formed about the upper inside surface of the trench. The formation of the collar oxide itself is a fairly complicated process. Typically, the top portion of the filled trench must be etched back leaving the dielectric layer along the upper portion of the trench sidewalls exposed. A conformal oxide layer (or typically a precursor thereof) is then deposited. The portion of the conformal oxide layer on the surface of the polysilicon (or other charge storage material) remaining in the partially filled trench is then selectively removed while leaving oxide about the upper sidewall of the trench to serve as a collar oxide. In the process of removing the conformal oxide from the polysilicon surface, the portion of this conformal oxide layer outside the trench (i.e. overlying the pad nitride) is also removed. The unfilled portion of the trench is then refilled with polysilicon to complete the formation of the capacitor plate with the collar oxide residing near the upper end of the trench.
This selective removal of the conformal oxide is a key step in trench capacitor fabrication. If the oxide is not completely removed from the polysilicon (or other fill material) surface where the trench is to be refilled, the performance of the capacitor may be compromised due to poor electrical connection between the polysilicon material deep in the trench and the polysilicon material deposited on trench refilling (i.e. in the portion of the trench having the collar oxide along the trench walls). Further, the selective removal of the conformal oxide must not adversely affect the underlying node dielectric layer, the dielectric pad dielectric layer or other features which present on the substrate outside the trench. Protection of the pad nitride layer near the top of the trench is especially important since it is typically used as an etch stop during the subsequent processing of the substrate. These process demands are made more difficult by the continued move to smaller dimension/higher aspect ratio geometries (e.g. 20 or higher).
Typically reactive ion etching or other dry etching processes have been used to achieve the selective removal of the conformal oxide. These processes have exhibited problems of slow etch rate (long process time), formation of undesired by-products/deposits, required use of problematic gases such as CO, and/or lack of reliability resulting in low yields or need for very tight control of process parameters. Thus, there remains a need for improved processes for conformal oxide etching, especially for use in the fabrication of trench capacitors.